An optical recording medium, such as a card type optical memory or a disc type optical memory, which has a high density data recording capability and is capable of being carriable, has recently attracted the attention of many people.
In the optical recording medium such as a card type optical memory, recording is done by forming optically (or magnetooptically) changed states according to data to be recorded at discrete spots on the surface thereof, and then a light beam, such as a laser beam, is projected thereonto to read the changed states.
Specifically, fine unevenness or light and dark patterns are provided on a surface of the optical memory card onto which a light beam is projected to recognize the recorded data according to the difference of reflectivity, refractive index, or transmissivity of the medium to the projected light beam. Alternatively, the recording medium may be photothermo-magnetically recorded and the recorded data may be read depending upon the change of polarization of the projected light due to the magnetooptical effect.
The optical memory card has a clock track for enabling synchronization with data reading/writing and a tracking guide line functioning as a reference for a light beam from a reading optical system to accurately trace a track. The track and line are provided for each of the data track in the conventional optical memory card and juxtaposed each other.
Heretofore, optical signals are detected by separate detectors for reading the data and clock bits, for effecting tracking and focussing in the reading of the data on the optical recording medium as described above. With this system, the optical system should inevitably be large and it is difficult to reduce the size and weight of a reading apparatus.